Anthelmintic resistance in parasites of small ruminants: sheep versus goats

Anthelmintic resistance among parasites of sheep and goats has been known to occur for at least four decades. Both species of host have similar genera of nematodes, but the nematodes in goat herds usually develop anthelmintic resistance more rapidly. In vitro tests show higher ED50 values in goats than in flocks of sheep. Sheep and goats differ in many ways; for example, goats have a higher metabolic rate and require higher dose rates for drugs. The immune system of goats is also different. Additionally, these animals are reared under different management systems, i.e. sheep graze pastures and goats browse bushes, and lambing/kidding periods are different. Most anthelmintics used in goats have not been licensed for this animal species, and correct dose rates have rarely been experimentally determined. Possible explanations for such differences are discussed.